The present invention relates to multimetal oxide materials of the formula I
(A)p(B)qxe2x80x83xe2x80x83(I),
where:
A is Mo12VaX1bX2cX3dX4eX5fX6gOx,
B is X71SbhHiOy,
X1 is W, Nb, Ta, Cr and/or Ce, preferably W, Nb and/or Cr,
X2 is Cu, Ni, Co, Fe, Mn and/or Zn, preferably Cu, Ni, Co and/or Fe,
X3 is Sb and/or Bi, preferably Sb,
X4 is Li, Na, K, Rb, Cs and/or H, preferably Na and/or K,
X5 is Mg, Ca, Sr and/or Ba, preferably Ca, Sr and/or Ba,
X6 is Si, Al, Ti and/or Zr, preferably Si, Al and/or Ti,
X7 is Cu, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr and/or Ba, preferably Cu, Ni, Zn, Co and/or Fe, particularly preferably Cu and/or Zn, very particularly preferably Cu,
a is from 1 to 8, preferably from 2 to 6,
b is from 0.2 to 5, preferably from 0.5 to 2.5,
c is from 0 to 23, preferably from 0 to 4,
d is from 0 to 50, preferably from 0 to 3,
e is from 0 to 2, preferably from 0 to 0.3,
f is from 0 to 5, preferably from 0 to 2,
g is from 0 to 50, preferably from 0 to 20,
h is from 0.1 to 50, preferably from 0.2 to 20, particularly preferably from 0.2 to 5,
i is from 0 to 50, preferably from 0 to 20, particularly preferably from 0 to 12,
x and y are each numbers which are determined by the valency and frequency of the elements in (I) other than oxygen and
p and q are each numbers which differ from zero and the ratio p/q is from 20:1 to 1:80, preferably from 10:1 to 1:35, particularly preferably from 2:1 to 1:3,
which contain the moiety (A)p in the form of three-dimensional regions A of the chemical composition
A: Mo12VaX1bX2cX3dX4eX5fX6gOx 
and the moiety (B)q in the form of three-dimensional regions B of the chemical composition
B: X71SbhHiOy 
where the regions A, B are distributed relative to one another as in a mixture of finely divided A and finely divided B, with the proviso that the multimetal oxide materials (I) are prepared using at least one separately preformed oxometallate B,
X71SbhHiOy,
xe2x80x83which is obtainable by preparing a dry blend from suitable sources of the elemental constituents of the oxometallate B which contain at least a part of the antimony in the oxidation state +5 and calcining said dry blend at from 200 to 1200xc2x0 C., preferably from 200 to 850xc2x0 C., particularly preferably from 250 to  less than 600xc2x0 C., frequently xe2x89xa6550xc2x0 C.
The present invention also relates to processes for the preparation of multimetal oxide materials (I) and their use as catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid.
WO 96/27437 relates to multimetal oxide materials which contain the elements Mo, V, Cu and Sb as essential components and whose X-ray diffraction pattern has the line of strongest intensity at a 2xcex8 value of 22.2xc2x0. WO 96/27437 recommends these multimetal oxide materials as suitable catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid. Furthermore, WO 96/27437 recommends using Sb2O3 as an antimony source for the preparation of these multimetal oxide materials. Preparation of an antimony-containing component beforehand is not described in WO 96/27437.
EP-B 235760 relates to a process for the preparation of Sb, Mo, V and/or Nb-containing multimetal oxide materials which are suitable as catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid. EP-B 235760 recommends using an antimony prepared beforehand as an antimony source for the preparation of these multimetal oxide materials.
The disadvantage of the multimetal oxide materials of the prior art is that their activity and the selectivity of the acrylic acid formation are not completely satisfactory when they are used as catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid.
It is an object of the present invention to provide novel multimetal oxide materials which, when used as catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid, have the disadvantages of the catalysts of the prior art to a reduced extent, if at all.
We have found that this object is achieved by the multimetal oxide materials (I) defined at the outset.
Very particularly preferred materials (I) are those whose regions A have a composition of the following formula (II)
Mo12Va,X1b,X2c,X5f,X6g,Ox,xe2x80x83xe2x80x83(II),
where
X1 is W and/or Nb,
X2 is Cu and/or Ni,
X5 is Ca and/or Sr,
X6 is Si and/or Al,
axe2x80x2 is 2 to 6,
bxe2x80x2 is 0.5 to 2.5,
cxe2x80x2 is 0 to 4,
fxe2x80x2 is 0 to 2,
gxe2x80x2 is 0 to 2 and
xxe2x80x2 is a number which is determined by the valency and frequency of the elements in (II) other than oxygen.
It is also advantageous if at least one of the moieties (A)p, (B)q of the novel multimetal oxide materials (I) is contained in the latter in the form of three-dimensional regions having the chemical composition A or B, whose maximum diameters dA and dB (longest connecting line between two points present on the surface (interface) of the region and passing through the center of gravity of the region), respectively, are from  greater than 0 to 300 xcexcm, preferably from 0.01 to 100 xcexcm, particularly preferably from 0.05 to 50 xcexcm, very particularly preferably from 0.05 to 20 xcexcm.
However, the maximum diameters can of course also be from 0.05 to 1.0 xcexcm or from 75 to 125 xcexcm (the experimental determination of the maximum diameter is permitted, for example, by a microstructure analysis by means of a scanning electron microscope (SEM)).
As a rule, the moiety (B)q is present in the novel multimetal oxide materials essentially in crystalline form, i.e. as a rule the regions B essentially comprise small crystallites whose maximum dimension is typically from 0.05 to 20 xcexcm. However, the moiety (B)q can of course also be amorphous and/or crystalline.
Particularly preferred novel multimetal oxide materials are those whose regions B essentially comprise crystallites which have the trirutile structure type of xcex1- and/or xcex2-copper antimony CuSb2O6. xcex1-CuSb2O6 crystallizes in a tetragonal trirutile structure (E.-O. Giere et al., J. Solid State Chem. 131 (1997), 263-274), whereas xcex2-CuSb2O6 has a monoclinically distorted trirutile structure (A. Nakua et al., J. Solid State Chem. 91 (1991), 105-112, or reference diffraction pattern in index card 17-284 in the JCPDS-ICDD index 1989). In addition, regions B which have the pyrochlore structure of the mineral partzite, a copper antimony oxide hydroxide with the variable composition CuySb2-x(O, OH, H2O)6-7(yxe2x89xa62.0xe2x89xa6xxe2x89xa61), are preferred (B. Mason et al., Mineral. Mag. 30 (1953), 100-112, or reference pattern in index card 7-303 of the JCPDS-ICDD index 1996).
Furthermore, the regions B may consist of crystallites which have the structure of copper antimony Cu9Sb4O19 (S. Shimada et al., Chem. Lett. (1983) 1875-1876 or S. Shimada et al., Thermochim. Acta 133 (1988), 73-77 or reference pattern in index card 45-54 of the JCPDS-ICDD index) and/or the structure of Cu4SbO4.5 (S. Shimada et al., Thermochim. Acta 56 (1982), 73-82 or S. Shimada et al., Thermochim. Acta 133 (1988), 73-77, or reference pattern in index card 36-1106 of the JCPDS-ICDD index).
Of course, the regions B may also consist of crystallites which constitute a mixture of the abovementioned structures.
The novel materials (I) are obtainable in a simple manner, for example by first separately preforming oxometallates B,
X71SbhHiOy,
in finely divided form as starting material 1. The oxometallates B can be prepared by preparing a preferably intimate, advantageously finely divided dry blend from suitable sources or their elemental constituents and calcining said dry blend at from 200 to 1200xc2x0 C., preferably from 200 to 850xc2x0 C., particularly preferably from 250 to  less than 600xc2x0 C., frequently xe2x89xa6550xc2x0 C. (as a rule for from 10 min to several hours). All that is essential to the invention is that at least a part of the oxometallates B of the starting material 1 (referred to below as oxometallates B*) is obtainable by preparing a preferably intimate, advantageously finely divided dry blend from suitable sources of the elemental constituents of the oxometallate B which contain at least a part of the antimony in oxidation state +5 and calcining said dry blend at from 200 to 1200xc2x0 C., preferably from 200 to 850xc2x0 C., particularly preferably from 250 to  less than 600xc2x0 C., frequently xe2x89xa6550xc2x0 C. (as a rule for from 10 min to several hours). The calcination of the precursors of the oxo-metallates B can generally also be carried out under inert gas, but also under a mixture of inert gas and oxygen, such as air, or under pure oxygen. Calcination under a reducing atmosphere is also possible. As a rule, the required calcination time decreases with increasing calcination temperature. Advantageously, the proportion of the oxometallates B* in the finely divided starting material 1 is at least 10, better at least 20, frequently at least 30 or at least 40, preferably at least 50, even better at least 60, particularly preferably at least 70 or at least 80, frequently at least 90 or 95, very particularly preferably 100, % by weight, based on the starting material 1.
Oxometallates B* are obtainable, for example, by the preparation methods described in detail in DE-A 24 076 77. Preferred among these is the procedure in which antimony trioxide and/or Sb2O4 are oxidized in an aqueous medium by means of hydrogen peroxide in an amount which is equal to or greater than the stoichiometric amount at from 40 to 100xc2x0 C. to give antimony(V) oxide hydroxide, aqueous solutions and/or suspensions of suitable starting compounds of the other elemental constituents of the oxometallate B* are added just before this oxidation, during this oxidation and/or after this oxidation, the resulting aqueous mixture is then dried (preferably spray-dried (inlet temperature: from 250 to 600xc2x0 C., outlet temperature: from 80 to 130xc2x0 C.)) and the intimate dry blend is then calcined as described.
In the process just described, for example, aqueous hydrogen peroxide solutions having an H2O2 content from 5 to 33 or more % by weight may be used. Subsequent addition of suitable starting compounds of the other elemental constituents of the oxometallate B* is recommended in particular when these are capable of catalytically decomposing the hydrogen peroxide. However, it would of course also be possible to isolate the resulting antimony(V) oxide hydroxide from the aqueous medium and to intimately dry-blend it, for example, with suitable finely divided starting compounds of the other elemental constituents of the oxometallate B* and then to calcine this intimate mixture as described.
It is important that the elemental sources of the oxometallates B, B* are either already oxides or are compounds which can be converted into oxides by heating, in the presence or absence of oxygen.
In addition to the oxides, particularly suitable starting compounds are therefore halides, nitrates, formates, oxalates, acetates, carbonates and/or hydroxides (compounds such as NH4OH, NH4CHO2, CH3COOH, NH4CH3CO2 or ammonium oxalate, which disintegrate and/or can be decomposed at the latest during calcination to give compounds which escape completely in gaseous form, may additionally be incorporated). For the preparation of oxometallates B, the intimate mixing of the starting compounds can generally be carried out in dry or in wet form. If it is effected in dry form, the starting compounds are advantageously used in the form of finely divided powders. However, the intimate mixing is preferably effected in wet form. Usually, the starting compounds are mixed with one another in the form of an aqueous solution and/or suspension. After the end of the mixing process, the fluid material is dried and is calcined after drying. The drying is preferably carried out by spray-drying.
After calcination is complete, the oxometallates B, B* can be comminuted again (for example by wet or dry milling, for example in a ball mill or by jet-milling) and the particle class, having a maximum particle diameter (as a rule from  greater than 0 to 300 xcexcm, usually from 0.01 to 200 xcexcm, preferably from 0.01 to 100 xcexcm, very particularly preferably from 0.05 to 20 xcexcm) in the maximum diameter range desired for the novel multimetal oxide (I) can be separated off from the resulting powder, frequently essentially comprising spherical particles, by classification to be carried out in a manner known per se (for example wet or dry sieving).
A preferred method of preparation of oxometallates B* of the formula (Cu,Zn)1SbhHiOy comprises converting antimony trioxide and/or Sb2O4 in an aqueous medium by means of hydrogen peroxide initially into a preferably finely divided Sb(V) compound, for example Sb(V) oxide hydroxide hydrate, adding an ammoniacal aqueous solution of zinc carbonate and/or copper carbonate (which may have, for example, the composition Cu2(OH)2CO3) to the resulting aqueous suspension, drying the resulting aqueous mixture, for example spray-drying it in the manner described, and calcining the resulting powder in the manner described, if necessary after subsequent kneading with water followed by extrusion and drying.
In the case of oxometallates B differing from oxometallates B*, it proves particularly advantageous to start from an aqueous antimony trioxide suspension and to dissolve therein the X7 elements as nitrate and/or acetate, to spray-dry the resulting aqueous mixture in the manner described and then to calcine the resulting powder in the manner described.
For the preparation of multimetal oxide materials (I), the starting materials 1 preformed as described can then be brought into intimate contact with suitable sources of the elemental constituents of the multimetal oxide material A
Mo12VaX1bX2cX3dX4eX5fX6gOx,
in the desired ratio and a dry blend resulting therefrom can be calcined at from 250 to 500xc2x0 C., it being possible to carry out the calcination under inert gas (e.g. N2), a mixture of inert gas and oxygen (e.g. air), reducing gases such as hydrocarbons (e.g. methane), aldehydes (e.g. acrolein) or ammonia, or under a mixture of O2 and reducing gases (e.g. all of the abovementioned ones), as described, for example, in DE-A 43 359 73. In the case of calcination under reducing conditions, it should be ensured that the metallic consituents are not reduced right down to the element. The calcination time is as a rule a few hours and usually decreases with increasing calcination temperature. As is generally known, all that is important with regard to the sources of the elemental constituents of the multimetal oxide material A is that either they are already oxides or they are compounds which can be converted into oxides by heating, at least in the presence of oxygen. In addition to the oxides, particularly suitable starting compounds are halides, nitrates, formates, oxalates, citrates, acetates, carbonates or hydroxides. Suitable starting compounds of Mo, V, W and Nb are also their oxo compounds (molybdates, vanadates, tungstates and niobates) or the acids derived from these.
The starting material 1 can be brought into intimate contact with the sources of the multimetal oxide material A (starting material 2) either in dry or in wet form. In the latter case, it is merely necessary to ensure that the preformed multimetal oxides B, B* do not go into solution. In an aqueous medium, the latter is usually ensured at a pH which does not differ too greatly from 7 and at xe2x89xa660xc2x0 C. and xe2x89xa640xc2x0 C., respectively. If said substances are brought into intimate contact in wet form, drying is then usually carried out to give a dry material (preferably by spray-drying). Such a dry material is automatically obtained in dry blending.
Possible mixing methods are thus, for example:
a. mixing a dry, finely divided, preformed starting material 1 with dry, finely divided starting compounds of the elemental constituents of the desired multimetal oxide A in the desired ratio in a mixer, kneader or mill;
b. preforming a finely divided multimetal oxide A by intimate mixing of suitable starting compounds of its elemental constituents (dry or wet) and then calcining the resulting intimate dry blend at from 250 to 500xc2x0 C. (regarding the calcination time, calcination atmosphere and element sources, statements made above are applicable); converting the preformed multimetal oxide A into finely divided form and mixing it with the finely divided starting material 1 in the desired ratio as in a.; in this mixing method, a final calcination of the resulting mixture is not essential;
c. stirring the required amount of preformed starting material 1 into an aqueous solution and/or suspension of starting compounds of the elemental constituents of the desired multimetal oxide A and then spray-drying the mixture; instead of the starting compounds of the elemental constituents of the desired multimetal oxide A, it is of course also possible to use a multimetal oxide A itself, already preformed according to b.
All mixing methods between a., b. and/or c. can of course also be used. The resulting intimate dry blend can then be calcined in the manner described and then shaped to give the desired catalyst geometry, or vice versa. In principle, the calcined dry blend (or possibly uncalcined dry blend where mixing method b. is used) can however also be used in the form of a powder catalyst.
Our own investigations have shown that, on calcination of the dry blend comprising the starting material 1 and the starting material 2, essentially no fusion of the components of the starting material 1 with those of the starting material 2 takes place and the structure type of the crystallites contained in the starting material 1 are often essentially retained as such.
As indicated above, this opens up the possibility, after milling of the preformed starting mixture 1, to separate off the particle class having the maximum particle diameter (as a rule from  greater than 0 to 300 xcexcm, preferably from 0.01 to 100 xcexcm, particularly preferably from 0.05 to 20 xcexcm) in the maximum diameter range desired for the multimetal oxide material (I) from the resulting powder, frequently comprising essentially spherical particles, by a classification to be carried out in a manner known per se (for example wet or dry sieving) and thus to use said particle class in a tailor-made manner for the preparation of the desired multimetal oxide material.
When the novel multimetal oxide materials (I) are used as catalysts for the gas-phase catalytic oxidation of acrolein to acrylic acid, the shaping to give the desired catalyst geometry is preferably carried out by application to preformed inert catalyst carriers, it being possible to effect the application before or after the final calcination. The conventional carrier materials, such as porous or nonporous aluminas, silica, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium silicate or aluminum silicate, may be used. The supports may be regularly or irregularly shaped, regularly shaped supports having pronounced surface roughness, for example spheres or hollow cylinders, being preferred. Among these, in turn, spheres are particularly advantageous. It is particularly advantageous to use essentially nonporous spherical steatite carriers with a rough surface, whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm. The layer thickness of the active material is advantageously chosen in the range from 50 to 500 xcexcm, preferably from 150 to 250 xcexcm. It should be pointed out here that, for coating the supports, in the preparation of such coated catalysts, the powder material to be applied is as a rule moistened and, after application, is dried, for example by means of hot air.
For the preparation of the coated catalysts, the coating of the supports is as a rule carried out in a suitable rotatable container, as previously disclosed, for example, in DE-A 2909671 or in EP-A 293859. As a rule, the relevant material is calcined before coating the carrier.
The coating and calcination process according to EP-A 293 859 can be used in a suitable manner known per se so that the resulting multimetal oxide active materials have a specific surface area of from 0.50 to 150 m2/g, a specific pore volume of from 0.10 to 0.90 cm3/g and a pore diameter distribution such that at least 10% of the total pore volume is associated with each of the diameter ranges from 0.1 to  less than 1 xcexcm, from 1.0 to  less than 10 xcexcm and from 10 xcexcm to 100 xcexcm. Moreover, the pore diameter distributions stated as being preferred in EP-A 293 859 may be established.
The novel multimetal oxide materials can of course also be operated as unsupported catalysts. In this respect, the intimate dry blend comprising the starting materials 1 and 2 is preferably compacted directly to give the desired catalyst geometry (for example by means of pelleting or extrusion), it being possible, if necessary, to add conventional assistants, for example graphite or stearic acid as lubricants, and/or molding assistants and reinforcing agents, such as microfibers of glass, asbestos, silicon carbide or potassium titanate, and calcined. Here, too, calcination can in general be effected prior to shaping. Preferred geometries for unsupported catalysts are hollow cylinders having an external diameter and a length of from 2 to 10 mm and a wall thickness of from 1 to 3 mm.
The novel multimetal oxide materials are particularly suitable as catalysts having high activity and selectivity (at given conversion) for the gas-phase catalytic oxidation of acrolein to acrylic acid. Acrolein produced by the catalytic gas-phase oxidation of propene is usually used in the process. As a rule, the acrolein-containing reaction gases from this propene oxidation are used without intermediate purification. Usually, the gas-phase catalytic oxidation of acrolein is carried out in tube-bundle reactors as a heterogeneous fixed-bed oxidation. Oxygen, advantageously diluted with inert gases (for example in the form of air), is used as an oxidizing agent in a manner known per se. Suitable diluent gases are, for example, N2, CO2, hydrocarbon, recycled reaction exit gases and/or steam. As a rule, an acrolein:oxygen:steam:inert gas volume ratio of 1:(1 to 3):(0 to 20):(3 to 30), preferably 1:(1 to 3):(0,5 to 10):(7 to 18), is established in the acrolein oxidation. The reaction pressure is in general from 1 to 3 bar and the total space velocity is preferably from 1000 to 3500 l(S.T.P.)/l/h. Typical multitube fixed-bed reactors are described, for example, in DE-A 2830765, DE-A 2 201 528 or U.S. Pat. No. 3,147,084. The reaction temperature is usually chosen so that the acrolein conversion in a single pass is above 90%, preferably above 98%. Usually, reaction temperatures of from 230 to 330xc2x0 C. are required in this respect.
In addition to the gas-phase catalytic oxidation of acrolein to acrylic acid, the novel products are however also capable of catalyzing the gas-phase catalytic oxidation of other organic compounds, in particular other alkanes, alkanols, alkanals, alkenes and alkenols, preferably with 3 to 6 carbon atoms (e.g. propylene, methacrolein, tert-butanol, the methyl ether of tert-butanol, isobutene, isobutane or isobutyraldehyde), to olefinically unsaturated aldehydes and/or carboxylic acids, and the corresponding nitriles (ammoxidation, especially of propene to acrylonitrile and of isobutene or tert-butanol to methacrylonitrile). The preparation of acrolein, methacrolein and methacrylic acid may be mentioned by way of example. However, they are also suitable for the oxidative dehydrogenation of olefinic compounds.
Unless stated otherwise, the conversion, selectivity and residence time are defined in this publication as follows:                                                         Conversion              ⁢                              xe2x80x83                            ⁢              C              ⁢                              xe2x80x83                            ⁢              of                                                                          acrolein              ⁢                              xe2x80x83                            ⁢                              (                %                )                                                        =                                                  no              .                              xe2x80x83                            ⁢              of                        ⁢                          xe2x80x83                        ⁢            moles            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            acrolein            ⁢                          xe2x80x83                        ⁢            converted                                              no              .                              xe2x80x83                            ⁢              of                        ⁢                          xe2x80x83                        ⁢            moles            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            acrolein            ⁢                          xe2x80x83                        ⁢            used                          xc3x97        100              ;                                                          Selectivity              ⁢                              xe2x80x83                            ⁢              S              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              the                                                                          acrylic              ⁢                              xe2x80x83                            ⁢              acid              ⁢                              xe2x80x83                            ⁢              formation              ⁢                              xe2x80x83                            ⁢              %                                          =                                                                                                        no                    .                                          xe2x80x83                                        ⁢                    of                                    ⁢                                      xe2x80x83                                    ⁢                  moles                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  acrolein                                                                                                      converted                  ⁢                                      xe2x80x83                                    ⁢                  into                  ⁢                                      xe2x80x83                                    ⁢                  acrylic                  ⁢                                      xe2x80x83                                    ⁢                  acid                                                                                                                          total                  ⁢                                      xe2x80x83                                    ⁢                                      no                    .                                          xe2x80x83                                        ⁢                    of                                    ⁢                                      xe2x80x83                                    ⁢                  moles                  ⁢                                      xe2x80x83                                    ⁢                  of                                                                                                      acrolein                  ⁢                                      xe2x80x83                                    ⁢                  converted                                                                    xc3x97        100              ;              Residence      ⁢              xe2x80x83            ⁢      time      ⁢              xe2x80x83            ⁢              (        sec        )              =                                                                      empty                ⁢                                  xe2x80x83                                ⁢                reactor                ⁢                                  xe2x80x83                                ⁢                volume                ⁢                                  xe2x80x83                                ⁢                filled                                                                                        with                ⁢                                  xe2x80x83                                ⁢                catalyst                ⁢                                  xe2x80x83                                ⁢                                  (                  l                  )                                                                                                                        Synthesis                ⁢                                  xe2x80x83                                ⁢                gas                ⁢                                  xe2x80x83                                ⁢                throughput                                                                                                          l                  ⁡                                      (                                          S                      .                      T                      .                      P                      .                                        )                                                  /                h                                                        xc3x97      3600.      